Torsional vibrations in a rotating shaft are manifested as a deviation from a uniform angular velocity and will, therefore, appear as phase, or equivalently, frequency modulation of any signal indicative of instantaneous angular velocity. Torsional vibrations are of particular concern to operators of large turbine-generators sets such as those used in the electric power generation industry. For these very large machines, even the smallest torsional vibration in the rotating shaft can cause damaging levels of stress, and may, in the most extreme case, cause shaft breakage and permanent damage to the machine itself. For example, the shaft of a large steam driven turbine-generator set may weigh several tons and may be severely damaged by a torsional vibration of only 0.01 degree. Obviously, it becomes highly desirable to be able to monitor the shaft torsional vibrations of such machines so that early corrective action can be taken.
For a turbine-generator connected to supply power to an electrical load, a fault on the connected power line produces, through the magnetic field interaction, a reaction torque on the rotating shaft which causes torsional vibrations within a band of frequencies at twice the machine running speed. These vibrations, for example, would appear at about 120 Hz. on the 60 Hz. machines employed in the United States.
Torsional vibration monitors useful for turbine-generators are, of course, now well known in the art as exemplified by the method and apparatus of Wolfinger taught in U.S. Pat. Nos. 3,885,420 and 4,148,222. However, in many instances, such as the above-mentioned cause of line fault produced torsional vibrations at 120 Hz., it becomes necessary to ascertain the spectral content of the torsional vibrations. Prior art techniques have suffered in this regard because of drift problems in the very narrow band-pass filters used to separate the spectral content of the torsional vibration signal. Significant uncertainty in the spectral analysis has resulted.
Accordingly, it is the principal objective of the present invention to provide stable, drift-free means by which the torsional vibrations of a rotating shaft may be monitored and analyzed for spectral content, and which means is particularly useful when applied as a monitor for a turbine-generator set.
Other objects, features, and advantages of the invention will become apparent from the following summary and detailed description of the invention.